1. Field of the Invention
The present invention relates to a light guiding device and in particular to a light guiding device operable to enable the combination of light from two separate sources into a single image.
2. Background of the Related Art
In certain circumstances it may be desirable to combine image information from two or more sources to provide a composite image. One such occasion is where it is desirable to provide text data (such as range or similar) over an image. Another such occasion is where it may be desirable to combine image data from an image intensifier (I2) with image data obtained from a thermal imager. An image intensifier amplifies visible light and near infra red light (say 400 nm-900 nm in wavelength); whilst a thermal imager operates in a further portion of the infra red light spectrum (say 7000 nm-14000 nm in wavelength). This is advantageous as the longer wavelengths are generally less attenuated by smoke and dust, although image intensifiers generally have better resolution than thermal imagers.
Current night vision systems using image intensification benefit from higher resolution and more natural imaging in comparison to imaging in the far infrared. However, I2 systems are limited in so far as they do need at least some ambient light to operate. A typical intensifier tube becomes noise limited at around 100 μlux which corresponds to a moonless overcast night sky. At this point and at lower light levels effective night vision requires infrared systems.
Thermal imaging also offers advantages over I2 in detecting targets concealed by camouflage. Addressing I2 limitations by including a thermal imaging capability improves overall night vision performance but at the expense of complexity and weight. Hand held systems are easy to interchange when each system is self contained with its own optics, display and power supply. On the other hand helmet or system mounted image intensifiers cannot be easily swapped over.
An example of an apparatus that provides this image combination is disclosed in US2008/0302966. In this system, a conventional image intensifier is provided with a clip on attachment comprising a thermal imager, a projector operable to project a visible image captured by the thermal imager and a light guiding device which directs light emitted by the projector into the aperture of the image intensifier. In this manner image from the thermal imager and the image intensifier may be combined to provide a single image.
The light guiding device comprises a reflector such as a prism, mirror, or similar provided within an opaque housing. The housing has a two fold purpose, both to provide sufficient structural integrity to hold the reflector in position and to shield the reflector from unwanted incident light. In order to provide optimal injection performance, the reflector is aligned with the optical axis of the image intensifier. The housing thus provides a significant obstruction to the field of view of the image intensifier. This can not only limit what can be seen by a user in terms of field of view but additionally may reduce the sensitivity of the image intensifier as it allows less visible light to enter the intensifier tube. Accordingly, the design of the opaque housing is a trade off between providing sufficient structural stability and minimising obstruction to other devices.
An additional problem with such systems is that the light emitted by the projector may be of sufficient intensity to swamp the captured light or that it is not clear what parts of the image are captured and what parts are a result of the projected light. In order to reduce this effect, the intensity of the projector can be adjusted manually. Whilst this does provide a solution to these problems, it is not optimal.
It is therefore an object of the present invention to provide an optical device that at least partially overcomes or alleviates the above problems.